The cylinder former was originally developed for papermaking, but is generally useful for forming fibrous webs from fiber slurries. It can be used as a standalone apparatus for forming a single ply sheet of fibers or in series to form a multiply web. Cylinder formers include a horizontally situated cylinder with a wire or plastic cloth surface that rotates in a vat containing a constantly refreshed dilute slurry of fibers, wherein the liquid carrying the slurry is typically water. The water associated with the slurry is drained through the cylinder and a layer of fibers is deposited on the wire or cloth. The drainage rate, in some designs, is determined by the slurry properties and water level inside the cylinder such that a pressure differential is formed. As the cylinder turns and water is drained, the fibrous layer that is deposited on the cylinder is peeled off of the wire or cloth and continuously transferred to a soft rubber couch roll. Further plys may be added, or additional treatments including heating or other means of drying the web are then employed depending on the ultimate intended end use. Cylinder formers are currently employed in the industry to form a variety of nonwoven fibrous webs. Wood based cellulose fibers are only one type of fiber that can be suitably dewatered to form a fibrous web; other natural fibers such as cotton, synthetic thermoplastic fibers such as polyolefin, polyester, or nylon fibers, inorganic fibers such as glass fibers, and the like may suitably employed to form fibrous webs using a cylinder former. Other materials, for example particles, latex-based binder resins, and the like are often included in slurries to form fibrous webs for a variety of industrially useful applications.
One important aspect of the construction of cylinder formers is the location and flow of the slurry as it is applied to the rolling cylinder. In some types of cylinder formers, the slurry is applied using a vat situated horizontally, such that the lower half of the cylinder is effectively immersed in the slurry. As the cylinder turns, fresh slurry is continuously pumped through the vat. A counterflow vat has slurry pumped into the vat such that the flow direction is opposite to the direction of the cylinder's rotation. A uniflow vat has slurry pumped into the vat such that that flow direction is the same as the direction of the cylinder's rotation. Each type of flow system has benefits and drawbacks that are well known to those of skill in the art. In another type of cylinder former, known as the “dry vat,” slurry is applied substantially vertically along the cylinder in the same direction as the cylinder's rotation. The area of the cylinder contacting the slurry, called the “forming area,” is restricted compared to that of other vat designs. Suction formers are dry vat type formers that have a very restricted forming area and utilize vacuum dewatering inside of the cylinder. The greater rate of water removal afforded by vacuum dewatering facilitates increased line speed relative to “gravity” type water drainage. Pressure formers are another dry vat type variation that employ a pressurized slurry instead of vacuum suction as a means to control the pressure differential.
In all of these constructions, single slurries are employed in single pass operations to form single ply fibrous layers of variable thickness. Multiply webs are formed by disposing more than one cylinder former in series, wherein as a fibrous mat is formed, it is combined with one or more additional mats formed on separate cylinder(s). In some cases, to form a multiply web, a first layer formed is couched on a second cylinder, and two layers are picked by another couch roll and transferred to a third stage cylinder. Each ply formed will have a distinct boundary, because each ply is completely formed prior to application of the next slurry or ply. However, for some applications it would be desirable to have a gradient of characteristics in transition from one ply to the next. For example, fibrous media having pore size gradients are advantageous for, among other applications, particulate filtration, where the filter otherwise can become clogged in the most upstream layers, thus shortening the lifetime of the filter. In some particulate filtration applications, it has been observed that the presence of interface(s) between layers of the filter element is where trapped particulate tends to build up. In some such applications, sufficient buildup between layers results in filter failure.
Additionally, fibrous media having a gradient of such characteristics as fiber chemistry, fiber diameter, crosslinking or fusing or bonding functionality, presence of binder or sizing, presence of particulates, and the like would be advantageous in many diverse applications. Such gradients can give rise to, for example, gradients in permeability, retention of particulates, pressure drop, species filtration, and the like when employed in filtration applications. Gradients of materials and physical attributes would be advantageous when provided through the thickness of a fibrous media, or over another dimension such as crossweb width or length of a fibrous media. Such gradients have not previously been known to be possible in conjunction with the ease of forming and compact design of a cylinder forming apparatus.
There is a need in the industry to provide a fibrous medium having a true gradient of materials, such as fibers of varying chemistry, diameter, aspect ratio, and the like using a cylinder forming apparatus. There is a need in the industry to provide a fibrous medium having a true gradient of other materials, such as resins, adhesives, crosslinkers, binders, particulates, and the like throughout the fibrous medium using a cylinder forming apparatus. There is a need in the industry for providing such gradients either through the thickness or the crossweb or downweb direction of a length of fibrous media using a cylinder forming apparatus. There is a need in the industry to form such constructions with sufficient ease and efficiency to make the products commercially and economically viable for a range of applications using a cylinder forming apparatus. There is a need in the industry to enable a gradient fibrous medium to be formed in single pass using a cylinder forming apparatus.